Project Summary Ischemic heart disease encompassing myocardial infarction (MI) and progression to heart failure (HF) is the leading cause of death in the United States. Significant cardiomyocyte (CM) death and limited ability to form new CM post-MI restricts repair leading to necrotic tissue expansion and eventual HF. Currently, the only available treatments for end-stage heart failure, heart transplant and left ventricular assist devices, are hindered by availability of donor hearts, limited medical resources and negative impacts on patients? quality of life. To improve available medical technology, injectable myocardial matrix (MM) hydrogels derived from decellularized porcine cardiac extracellular matrix have demonstrated promising pre-clinical results including significantly improved cardiac function, increased cardiac muscle and reduced negative left ventricular remodeling in animal models post-MI. The goal of this project is to determine whether increases in new CM formation occurs post-MI with MM therapy that could contribute to the observed therapeutic outcomes of improved cardiac muscle mass and function in treated groups. Since the heart has limited native repair and CM turnover, it is currently debated whether new CM formation can be significantly induced from pre-existing CMs and/or endogenous progenitor cells to repair the heart. We hypothesize that MM treatment induces formation of new CMs from pre- existing CMs and/or endogenous progenitor cell populations. Utilizing DNA labels and transgenic models to track CM response in vivo, the following aims are designed to determine whether CM formation is a contributing mechanism to MM cardiac repair. ? Specific Aim #1: Investigate whether myocardial matrix increases DNA repair and/or proliferation of cardiomyocytes post-myocardial infarction ? Specific Aim #2: Utilize transgenic mouse models to determine whether myocardial matrix therapy induces pre-existing cardiomyocytes or endogenous progenitor cells to contribute to new cardiomyocyte formation ? Specific Aim #3: Analyze nuclei content and transcriptome of labeled cardiomyocytes to provide evidence of new cardiomyocyte formation from myocardial matrix therapy The long term objective of this proposed research is to determine cellular mechanisms involved in MM induced repair and use this understanding to create more complete biomaterial treatments for cardiovascular diseases. This project will advance our basic understanding of cellular mechanisms that can combat chronic cardiac tissue diseases and contribute to improving the nation?s cardiovascular health, which supports the mission goals of the National Heart, Lung and Blood Institute.